This invention relates to apparatus and method for removing substances such as drugs and toxins from whole blood.
The incidence of end-stage renal disease (ESRD) in the United States exceeds 200 per million population and continues to rise by approximately 8% per year, largely with the inclusion of elderly hypertensive and/or diabetic patients (see, for example, "Morbidity and Mortality of Dialysis," NIH Consensus Development Conference Statement, Nov. 1-3, 1993, 11:1-33). The treatment of choice for ESRD is kidney transplant. However, the limited number of donor organs, presence of qualifying co-morbidities, and low transplant rate for the elderly, ensures that hemodialysis will remain the primary method of renal replacement therapy in the foreseeable future. By the turn of the century, nearly 390,000 patients are expected to be enrolled in dialysis programs.
It is often necessary or desirable to remove substances from blood without damaging the cells contained within whole blood. An example is the desirability of removing the anticoagulant heparin from blood before the blood is returned to a dialysis patient. A well known enzyme, heparinase, reacts with heparin causing it to break down. Unfortunately, heparinase immobilized on solid supports is unsuitable for use with prior art bioreactors to remove heparin from blood, because the solid supports cause hemolysis in whole blood. A goal has been to design a bioreactor which would allow the clinical use of enzymes such as heparinase in a safe and efficient manner with whole blood. Conventional membrane reactors (hollow fibers) have the disadvantage of reduced mass transfer to a reactive membrane and therefore surface area of contact has to be maximized often resulting in infeasible devices. Fluidized beds of, for example, agarose beads result in good mass transfer but the fluid dynamics of the particles in blood can lead to hemolysis of the red cells and packing of the beads will occur at high flow rates.
Another substance which it is highly desirable to remove from the blood of dialysis patients is .beta..sub.2 -microglobulin, which has been implicated in dialysis related amyloidosis (DRA). DRA is a severely debilitating, potentially life-threatening, and inevitable consequence of long-term hemodialysis. It is characterized by amyloid deposits preferentially in osteoarticular locations. The clinical sequela includes chronic pain, carpal tunnel syndrome, joint deformities, destructive arthropathy, cystic bone lesions, fractures, and spondylarthropathy. The incidence and severity of DRA increases with the length of dialysis therapy, reaching 70% after 10 years and 100% after 20 years. Further information about DRA can be found in Drueke, et al, "Dialysis-associated amyloidosis," Advan. Renal Replac. Therapy (1995) 2:24-39, and Schaeffer et al, "Pathogenetic and diagnostic aspects of dialysis-related amyloidosis," Nephrol. Dial. Transplant (1995) 10(Suppl 3):4-8. DRA is the most incapacitating consequence of long-term dialysis and remains inescapable.
In the absence of established methods to treat or avoid DRA, interventions are limited to merely symptomatic relief. Patients are typically on chronic pain management with variable efficacy. Low-dose steroids relieve some of the symptoms but often have intolerable side-effects. Numerous orthopedic procedures are the rule, including carpal tunnel surgery, arthroscopic synovectomy, bursectomy, cyst curettage and filling, tendon sheath and ligament resections, and others. For advanced destructive arthropathy, prosthetic replacements of weight-bearing joints, such as the hips, are commonly necessary. In patients with destructive spondylarthropathy associated with spinal cord or nerve-root compression, orthopedic consolidation may constitute a life-saving procedure. Conservative and approximate estimates of the cost to treat the complications of DRA exceed $500 million/year.
Although the mechanisms of DRA development are under investigation, it is certain that retention of .beta..sub.2 M, the precursor molecule of the amyloid fibrils, is an absolute prerequisite. .beta..sub.2 M is a non-polymorphic single chain polypeptide (11,800 Mw) that is continually shed from cell membranes. It is a noncovalently-bound subunit of the class I Major Histocompatibility Complex. It accumulates in renal failure primarily due to diminished excretion; it has been estimated that normal kidneys remove 150 mg of .beta..sub.2 M per day. Typical .beta..sub.2 M levels in normal adults are in the range of about 1-3 mg/l blood, while renal failure levels may reach or exceed 60 mg/l blood. Chronically elevated levels of .beta..sub.2 M must be present for DRA to occur. This causal role is further supported by demonstrations that measures effective at reducing the supply of .beta..sub.2 M can slow or halt disease progression and improve symptoms. These findings strongly support the development of a system able to re-establish and maintain normal levels of .beta..sub.2 M with the efficiency currently only achievable with a normally functioning kidney.
Conventional hemodialysis systems, which consist of a regenerated cellulosic membrane (cuprophane), do not remove .beta..sub.2 M. Although inexpensive, this membrane is impermeable to proteins, such as .beta..sub.2 M, resulting in a progressive accumulation of this protein. Therefore, research efforts have focused on modifying the dialysis technique to lower circulating .beta..sub.2 M levels, in the hope of favorably impacting the occurrence rate and course of this disease.
The use of highly permeable, synthetic dialysis membranes, such as polyacrylonitrile (AN69) and polysulfone (F60) have been shown to lower levels of .beta..sub.2 M, especially when used in conjunction with hemofiltration or hemodiafiltration. Although substantial reductions can be attained, .beta..sub.2 M removal is still incomplete and accumulation of the remaining protein continues to contribute to the pathophysiology of DRA. An additional concern with these systems is the indiscriminate removal of other middle weight molecules, such as hormones and growth factors. Given the performance limitations, these dialyzers are prohibitively expensive for use in a government regulated, capitated system, such as hemodialysis.
Hemoperfusion methods involving extracorporeal adsorption columns have also been used in conjunction with synthetic membrane hemodialysers to remove .beta..sub.2 M. The most developed of these is the BM-01 column containing a hexadecyl alkyl compound which has recently been tested in small groups of patients. Although superior to high-flux hemodialysers alone, protein accumulation still occurs, making the expense of this combination approach impractical. This column also requires a large priming volume, which may complicate the dialysis management of cardiovascularly tenuous patients, particularly since adsorption columns tend to have a high pressure drop along the length of the column. Further, this treatment has been found to induce hypotension, a reduction in hematocrit and platelet count, and the loss of beneficial plasma proteins (mainly lysozyme). Other adsorbents that have been studied in vitro include collagen, gelatin, and various ligands. However, non-specific binding remains a concern with these approaches as none of them is geared specifically toward reduction of .beta..sub.2 M level.
Immunoadsorption approaches are attractive for their superior specificity and affinity as was demonstrated with the application of high-performance immunoaffinity chromatography to the removal of .beta..sub.2 M. Unfortunately, this system is not compatible with whole blood and requires plasma separation by filtration or centrifugation before the treatment can be applied. This is not a reasonable procedure to impose on patients already debilitated by extensive chronic interventions. An alternative immunoadsorption method is the immunoaffinity column in which antibodies are immobilized onto insoluble supports. This system is also unusable with whole blood. Furthermore, the flow velocity through packed columns is limited, resulting in a relatively slow removal process. The development of hemocompatible immunoadsorbents from conventional hemodialyzers has also been proposed. Although a bifunctional device has obvious appeal, the design limitations of this technology make it inadequate for this application. The dialyzers offer a finite, and relatively small, surface area for antibody immobilization, which is most suited for removing circulating compounds present in low concentrations. This would not be sufficient to counterbalance the 1,400-2,000 mg of .beta..sub.2 M generated weekly. In addition, compared to traditional particulate matrices, antibody binding to cuprophan-based dialyzers is less stable. Therefore, a large amount of ligand is required to achieve satisfactory immobilization. This also presents concerns of antibody shedding into the circulation, deteriorating binding capacity of the system, and a potential for unwanted immunological side effects. The modified dialyzers also have decreased clearance values and increased ultrafiltration coefficients. The performance limitations and cost of these systems have remained substantial obstacles to their establishment as common clinical modalities.
Pharmacological interventions are also being explored. Low molecular weight anionic sulphonate and sulphate compounds designed to interfere with amyloid-basement membrane interactions show promise in the animal model of familial amyloidosis, although the relevance to DRA has not been investigated. In addition, aminoguanidine, which prevents the glycation of .beta..sub.2 M thought important in amyloidogenesis, is also being considered.
There is currently no renal replacement strategy that can achieve the same efficiency of .beta..sub.2 M removal as a normally functioning kidney. However, the approaches attempted thus far have provided invaluable information regarding the clinical benefits of even a partial reduction in protein load. The favorable impact on the onset and course of DRA supports the development of a system able to re-establish and maintain normal levels of .beta..sub.2 M.
It is an object of the present invention to provide a bioreactor capable of removing heparin, .beta..sub.2 M, or other substances from plasma with minimal or no damage to blood cells. This and other objects are achieved by the apparatus and methods described and claimed hereinbelow.